fix: resolve Next.js hydration mismatches for browser APIs

[Abhishek2005-ard]

Description

This PR resolves two React Hydration Mismatch errors that were triggering the Next.js error overlay in development.

The mismatches occurred because the application was directly evaluating browser-specific APIs (localStorage and navigator.platform) during the initial render cycle. Because these APIs either do not exist or evaluate differently during Server-Side Rendering (SSR), the server-rendered HTML did not match the initial client hydration payload.

Changes Made

• useVideoEditor.ts: Refactored the soundOnCompletion initialization. It now initializes cleanly to its default value and uses a useEffect to safely read and apply the user's saved preference from localStorage post-hydration.
• VideoEditor.tsx: Converted the isMac constant into a state variable and moved the navigator.platform check into a useEffect hook to prevent text node mismatches between "Ctrl" + Enter (server) and "⌘" + Enter (Mac client).

Testing Instructions

1. Run npm run dev to start the Next.js development server.
2. Open the application in your browser.
3. Verify that the red "1 Issue" error overlay no longer appears in the bottom left corner.
4. If testing on a Mac, verify that the keyboard shortcut correctly updates to display "⌘" + Enter after the initial page load.

close #1387

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[github-actions]

👋 Thanks for your PR, @Abhishek2005-ard!

Welcome to Reframe — a browser-based video editor built for everyone 🎬

What happens next

1. 🤖 Automated checks — build & TypeScript typecheck will run automatically
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Quick checklist

• PR title follows Conventional Commits (e.g. feat: add dark mode)
• Linked the issue this PR closes (e.g. Closes #123)
• Tested the changes locally (bun run dev)
• Build passes (bun run build)

Useful links

• 📖 CONTRIBUTING.md
• 🐛 Open Issues
• ⭐ Star the repo to support the project!

Happy coding! 🎉

[github-actions]

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